The present invention relates to an apparatus for detecting misfires in the cylinders of an internal combustion engine on a vehicle.
Generally, in a multi-cylinder type engine, combustion variation is caused by the following reasons:
(1) Complexity of the shape of the intake pipe;
(2) Uneven distribution of intake air caused by intake air interference. Intake air interference refers to the interference of compression waves of intake air between the intake ports and a converged part of the intake manifold.
(3) Combustion temperature differences between the cylinders; and
(4) Variations in the volume of the cylinder's combustion chamber and variations in the shape of the pistons generated during the machining of tho pistons.
In recent high-performance engines, which generate high power and have low fuel consumption, deterioration or malfunction of injectors and ignition plugs often causes variations in combustion among the cylinders. The combustion variation may result in intermittent misfires.
Misfires not only cause the engine torque to fluctuate, but also discharge unburned fuel from the misfiring cylinder and leak unburned fuel out of the engine. It is therefore necessary to detect misfires and to inform the driver of the occurrence of a misfire as soon as the misfire occurs. This allows the driver to deal with misfires as early as possible. Several techniques for quickly detecting misfires and informing a driver of the misfires have been proposed.
One of the proposed techniques includes an engine crankshaft having a rotor for detecting misfires. The rotor has plurality of teeth that project from the circumferential surface. Equal intervals are provided between each pair of adjacent teeth. A sensor is opposed to the circumferential surface of the rotor. The sensor detects passage of each tooth. When the rotor is rotating, the time period during which the rotor rotates by a certain number of degrees is computed. A determination value is computed based on the computed time period. Misfires are detected if the determination value is greater than a determination level.
The teeth on the rotor may be displaced. The interval between each pair of tooth should be equal. However, the location of each tooth will be slightly displaced from where it should be if the machining of the teeth is not as accurate as required. This displacement of teeth location is referred to as teeth displacement. Teeth displacement as used herein includes axial displacement. Axial displacement is displacement of the teeth along the axis of the crankshaft. One of the causes for the axial displacement is crankshaft torsion. When determining that determination level, the teeth displacement must be taken into consideration. In the prior art apparatuses, the determination level is learned and renewed for adequately adjusting the determination level.
For example, Japanese Unexamined Patent Publication No. 4-265475 discloses an apparatus that detects a cylinder in which combustion has been stopped by clogging or malfunction of the corresponding injector. The apparatus compensates for teeth displacement when the supply of fuel to the engine is temporarily stopped, or when a fuel cut-off operation is executed. Since fuel is not combusted during a fuel cut-off operation, detection of the teeth displacement is not affected by misfires. During a fuel cut-off operation, a test cylinder is selected. Then, the time during which the associated piston moves from one point to another is computed. In other words, the piston speed is computed in the test cylinder.
In the same manner, the piston speed is detected in another two cylinders, or reference cylinder, the combustion strokes of which are before and after the combustion stroke of the test cylinder. The average piston speed of the reference two cylinders is computed. A compensation value is computed based on the difference between the piston speed of the test cylinder and the average piston speed of the reference cylinders. A misfire determination level of the test cylinder is computed based on the piston speed of the test cylinder by referring to function data stored in a memory. The determination level is then compensated with the above computed compensation value and stored in the memory. The compensated determination level is thus learned.
Thereafter, when the fuel cut-off operation is finished and fuel injection is started again, a determination value for detecting the combustion state of the test cylinder is computed based on the piston speed in the test cylinder and the average piston speed of the reference cylinders. If the determination value is smaller than the determination level, it is judged that a misfire has occurred in the test cylinder.
As described above, a misfire determination level is learned, or compensated and stored in the memory, when a fuel cut-off operation is executed. This eliminates errors caused by teeth displacement of the rotor when misfire detection is performed. The accuracy of misfire detection is thus improved.
In the above described prior art, the determination level is learned, or compensated and stored in the memory, only when a fuel cut-off operation is performed. Generally, a fuel cut-off operation is performed when the vehicle speed is in a certain range, the crankshaft speed is in a certain range and an idle switch is on (when the throttle valve is fully closed). However, such a fuel cut-off operation is not often performed when the vehicle is moving. This results in the determination level for each cylinder being rarely learned, or rarely compensated and stored in the memory.
Further, even if a fuel cut-off operation is performed, it generally lasts for very short time. Therefore, there is not sufficient time for learning the determination level. When the learning of the determination level is rarely performed, the accuracy of the misfire detection is degraded.